Band pass amplifier coupling



-5i 1937- I R; A. BRADEN 2,066,875

7 BAND {PASS AMPL FIER COUPLING Original Filed Dec. .12, 1929 2 Sheets-Sheet 2 INVENTOR RENE A BRADEN ATTORN EY Patented Jan. 5, 1937 PATENT OFFICE,

BAND PASS AMPLIFIER COUPLING Rene A. Braden, Mcrchantviile, N. J assignor to Radio Corporation of America, a corporation of Delaware Original application December 12, 1929, Serial No. 413,450. Divided and this application September 16, 1932, Serial No. 633,441

6 Claims.

My present invention which is a division of application Serial No. 413,450, filed December 12, l929and issued as U. S. Patent No. 2,037,614, relates to amplifiers, and more particularly, to a band pass amplifier adapted to operate with a constant accepted band width.

In a co-pending application Serial No. 278,105 filed May 16, 1928, I have disclosed band pass amplifiers of the type comprising two or more circuits, each tuned tothe mid-band frequency, and coupling means for the circuits arranged so as to produce a constant accepted band width. It was further shown, in the said application, that to produce this latter condition the coupling between the tuned circuits must vary as the the type described in the above mentioned application. The method of coupling tuned circuits employs a single means, the latter being variable and linked tothe tuning control in such a manner that, as the circuits are tuned for various wave frequencies, the coupling will be caused to vary in the proper way to maintain constant band width.

Accordingly, it is one of the main objects of the present invention to provide a method of, and means for, maintaining constant the band width passed in an amplifier which consists in variably coupling the primary and secondary tuned circuits of the amplifier, and linking the single coupling control to the tuning control so as to maintain said band with constant independently of the wave frequency to which the circuits are tuned.

Other objects of the invention are to improve generally the simplicity and efficiency of band pass amplifier coupling arrangements, and to provide devices of the latter type which are not,

only reliable in operation, but economical an simple to manufacture.

The novel features which I believe to becharacteristic of my invention are set forth in particularity in theappended claims, the invention itself, however, as to both its organization and method of operation will best be understood by series with the condensers and 6. are tuned to the mid-band. wave frequency by varying the inductances 3 and 1.

reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 shows diagrammatically anovel type of coupling arrangement,

Figs. 2 to 9 inclusive show modified coupling arrangements embodying the present invention.

Referring, now, to the accompanying drawings in which like characters of reference indicate the same elements in the different modifications, the numeral I indicates an electron discharge tube of either the three electrode or four electrode type, and shown preferably as the latter, the incoming signal energy to be amplified being impressed between the grid and cathode of the tube and designated as Input; The reference numerals 3 and 4 denote an inductance and condenser respectively thus forming the primary tuned circuit which is tuned to the mid-band frequency of the incoming signal.

The numerals 6 and I represent a condenser and inductance respectively, and form the secondary tuned circuit, which is also tuned to the mid-band frequency. The numeral 8 designates a second electron discharge device, preferably of the four electrode type, whose grid is con- .nected to receive the voltage which is developed across the secondary circuit 6, I. The term Output, in each modification, represents the circuit to which any subsequent circuits are connected toemploy the amplified output of the device 8.

It is to be understood, that the circuits embodying my invention are intended to select a definite band of frequencies from any part of a relatively wide range of frequencies. For example, a broadcast receiver embodying the invention would be capable of selecting and amplifying all signal currents contained within a band 10 kilocycles in width and would be able to select this 10 kilocycle band from any part of the frequency spectrum between 550 kilocycles and 1500 kilocycles. The mid-band frequency in'this case is equal to. the carrier wave frequency, and

the accepted bandextends equally on each side of the carrier frequency.

Referring now to Fig. l in particular, the primary tuned circuit consists of the inductance coil 3 and the condensers 4 and 5 in series, and the secondary circuit consists of the coil 1 in The circuits The circuits are coupled by the condenser 5, and variation of the coupling is effected by varying the capacity of 5. The variable inductances 3 and I and the condenser 5 are linked together mechanically in any suitable manner, as indicated by I2. The rate of variation of the coupling condenser 5 is so chosen that the band pass net-work transmits to the tube 8, thruout the frequency spectrum between 550 and 1500 kcs., a bandof 10 kcs. wide. The particular mechanical coupling between the tuning elements and the coupling element is not important, and depends upon the nature of the circuit employed as well as upon the desires of those skilled in the art. The essential object of this invention is to provide a means for mechanically and simultaneously adjusting the coupling reactance with the tuning reactances of the band pass networkin such a way that the selected band is the same width over the entire tuning range of the network. It will be noted that an'inductance 2, in the plate circuit of 1 the device I, is coupled, as at M, to the inductance 3 and functions to transfer energy to the primary circuit, 3, 4 and 5 from the device I.

In the modification shown in Fig, 2, it will be observed that the primary and secondary circuits are tuned by varying the condensers 4 and 6. The output terminals of the tube I are connected directly to the tuned circuit 3, 4, 5. Coupling condenser 5 is, again, arranged to be controlled simultaneously with the condensers 4 and Bby a single tuning control I2.

The circuit shown in Fig. 3 is derived from that shown in Fig. 2, by interchanging the positions of inductance 3 and condenser 4; inductance I and condenser 6 being similarly interchanged. In addition, a capacity 3' is connected in series between the anode of device I and inductance 3, while the said anode is energized from a source B through an inductance 4'. In Figs. 1, 2 and 3, I have shown three alternative methods of coupling the anode circuit of a tube to the tuned circuit following. Usually any of these methods can be used with any circuit if the tube is a screen grid tube, but with a three electrode tube the circuit of Fig. 1 is preferable.

Fig. 4 shows a circuit employing a novel type of variable coupling condenser, oneform of which has been described in my copending'application, Serial No. 278,105, filed May 16th, 1928. In this circuit the coil 3 and the variable condenser 4 constitute the primary circuit, the coil 1 and variable condenserfi the secondary circuit and the primary and secondary circuits are coupled by the electrostatic capacity between the plates 9 and In which are attached to the high potential ends of the condensers 4 and 6, respectively.

A metal plate II is connected to ground (i. e. to the filament side of the circuit) and is mounted so that it can be slid between the plates 9 and I0 or drawn out. It must be insulated from the plates 9 and I0, and should move in a plane parallel to them and half way between. The capacity between 9 and I0 is reduced by the plate II, the amount of reduction being roughly proportional to the area of II which is between the plates 9 and I0. a

By this means the coupling capacity can be reduced from a comparatively large value almost to zero, and the ratio of maximum to minimum capacity is much greater than canbe secured with the conventional two-plate variable condenser of equal maximum capacity. Instead of being mounted as indicatedin .the drawings, the

' The capacities of these two condensers are controlled by the grounded plates II and II. This circuit is especially convenient when the condensers 4 and 6 are separated by a shield par- Ltition 22, because-of the symmetrical arrangement with respect to the shield; and additionally, .in View of the fact that the connections between condensers 4 and 6 and the coupling condensers'do-not pass through the shield.

Of course, the plates II and I I' are actuated with the rotors of condensers 4 and 6 in a manner similar to the actuation of plate .II and the rotors of condensers 4, 6 as shown inFig. 4.

In the modification shown in Fig. 6 the secondary tuned circuit consists of the coils I and I3 and the variable condenser Bin series. The coil I3 is inductively coupled to the coil 3, the amount of mutual inductance M being controlled by changing the position of inductance I3 with respect to inductance 3. The coupling control is linked to the tuning control I2.

The modified coupling arrangement in Fig. 7 embodies the link circuit coupling devices described in my co-pending application Serial No. 390,468, filed September 5, 1929. The inductance 3, of the primary circuit, is tuned by the condenser 4 and coupled to the link circuit by the inductance I5. The latter is connected in series. with a variable inductance II, and a second coupling inductance I6, the latter being coupled to the inductance 'I of the secondary circuit. (The coupling between the primary and secondary circuits. is, thus, controlled by mechanically connecting the means for varying the inductance II, with the rotors of tuning condensers 4 and 6. l

The circuit shown in Fig. 8 is similar to the arrangement in Fig. '7, except for the fact that a variable coupling condenser I8 is employed instead of the variable inductance I I.

In Fig. 9 there is shown a capacity-coupled circuit, the primary circuit 4, 3 being coupled to the secondary 6, I by the variable condenser I9. The three variablecondensers are varied simultaneously by the control I2. As explained in con,-

nection with Fig. l, the rate of variation of the 7 coupling reactanceswith tuning in each of Figs. 2 to 9 is such that the band width selected is constant over the entire tuning range. In each of the constructions shown in Figs. 2 to 9 the requirementsof the particular case will determine will be apparent to one skilled in the art that my invention is by nom'eans limited to the particular organizations shownand described, but that many modifications maybe made without departing from the scope of my invention as set forth in the appended claims.

- What I claim is:

1; A band pass amplifier comprising at least two tuned circuits, an electrostatic shield between said circuits, at variable .couplingreactance positioned between one of said tuned circuits and the shield, a second variable reactance connected to the first reactance and disposed between the shield and the other tuned circuit, said reactances varying the coupling between the tuned circuits simultaneously with tuning adjustments so as to maintain the accepted band width constant.

2. A band pass amplifier comprising at least two tuned circuits, an electrostatic shield between said circuits, two coupling condensers connected in series between said circuits, one being placed on each side of the electrostatic shield, and a grounded adjustable member associated with each coupling condenser and arranged to be moved into or out of the space between the condenser plates, said coupling condensers varying the coupling between the tuned circuits simultaneously with tuning adjustments so as to maintain the accepted band width constant.

3. In combination, in a tunable band pass filter circuit, an oscillation circuit including an inductance coil and a variable tuning condenser, a second oscillation circuit including a coil and a second variable tuning condenser, a common means for simultaneously and similarly varying said condensers to tune both said oscillation circuits to a common desired modulated carrier frequency of the broadcast range, an untuned adjustable reactance common to both said oscillation circuits comprising the sole coupling means between them and providing suificient coupling therebetween to impart a band pass characteristic to the filter circuit of a width equal to the modulation side bands of said carrier frequency, said reactance comprising at least one variable condenser connecting points of high alternating potential of said oscillation circuits, means for mechanically coupling a variable element of said adjustable reactance to said common means whereby said reactance is adjusted to vary said coupling as said oscillation circuits are tuned, the rate of variation of said reactance being chosen to maintain the said width of the said characteristic substantially constant throughout the tuning range of said variable condensers.

4. In combination, in a tunable band pass filter circuit, an oscillation circuit including an inductance coil and a variable tuning condenser,

a second oscillation circuit including a coil and a tion circuits comprising the sole coupling means between them and providing suflicient coupling therebetween to impart a band pass characteristic to the filter circuit of a width equal to the modulation side bands of said carrier frequency, said reactance comprising at least one condenser having a pair of stator plates connected to similar high frequency potential points of said oscillation circuits and an adjustable plate connected to a point of lesser high frequency'potential value, means for mechanically coupling said adjustable plate to said common means whereby said reactance is adjusted to vary said coupling as said oscillation circuits are tuned, the rate of variation of said reactance being chosen to maintain the said width of the said characteristic substantially constant throughout the tuning range of said variable condensers.

5. The combination of two alternating current circuits and a pair of spaced planar metallic plates connected to high alternating voltage points of said circuits, and at least one metallic plate at a relatively lower alternating voltage disposed in a plane intermediate the planes of said pair of plates, said three plates providing coupling between the circuits and said intermediate plate being adjustable with respect to the other two plates to vary the magnitude of said coupling.

6. In combination, in a tunable band pass filter circuit, an oscillation circuit including an inductance coil and a variable tuning condenser, a second oscillation circuit including a coil and a second variable tuning condenser, a common means for simultaneously and similarly varying said condensers to tune both said oscillation circuits to a common desired modulated carrier frequency of the broadcast range, coupling means connected to both said oscillation circuits comprising the sole coupling means between them and providing sufficient coupling therebetween to impart a band pass characteristic to the filter circuit of a width equal to the modulation side bands of said carrier frequency, said coupling means comprising at least one condenser having a pair of stator plates and an adjustable plate disposed intermediate the stator plates, means for mechanically coupling said adjustable plate to said common means whereby said coupling means is adjusted to vary said coupling as said oscillation circuits are tuned, the rate of variation of said coupling means being chosen to maintain the said width of the said characteristic subtantially constant throughout the tuning range of said variable condensers.

RENE A. BRADEN. 

